Flat Bonneville-type spoiler

[brucepick]

This version of the template drawing shows two ways to meet the template without going past the rear bumper.

Aero wizards - what do you see as the plusses/minuses of these two?

Here are my poorly educated guesses:
Blue line is intended to be horizontal. Would allow a fairly clean view out the rear window. I think I prefer this version but consider the purple line one also.

Purple line would add downforce?? And I believe generating that downforce creates its own additional amount of drag? Or are the added downforce and drag very small due to relatively low pressure in the area just above the trunk lid?

[California98Civic]

Originally Posted by brucepick

This version of the template drawing shows two ways to meet the template without going past the rear bumper.

Aero wizards - what do you see as the plusses/minuses of these two?

Here are my poorly educated guesses:
Blue line is intended to be horizontal. Would allow a fairly clean view out the rear window. I think I prefer this version but consider the purple line one also.

Purple line would add downforce?? And I believe generating that downforce creates its own additional amount of drag? Or are the added downforce and drag very small due to relatively low pressure in the area just above the trunk lid?

Visibility out the window is essential, and your purple line would block more than a Kamm back, probably. A horizontal spoiler would be something like the XR4TI double-spoiler thing of yore (unique look). I'm still looking beyond the bumper. I have another build of the spoiler good to go. I'll test it soon, maybe tomorrow or Wednesday. Hope I don't get a fix it ticket. I'll post photos in a minute.

[brucepick]

We're up to our eyeballs in home repairs right now. Hopefully some time this summer I'll get to build and test some kind of rear spoiler design(s).

[California98Civic]

Unlike some of you, I am still looking for the template out beyond the rear bumper. I also wanted endplates because I suspect they help with vortices off the end of the car. Below you can see a photo of the new effort as well as a profile photo template of how it compared to plan yesterday. Before building the endplates, I reduced the angle to the red line below, extending it also by two inches. This build extends 15 inches off the trunklid, probaly 11-12 inches behind the bumper.

EDIT: the wrong template image uploaded, but basically the new spoiler just about meets the red line in the image--length and angle.

[cfg83]

brucepick -

Originally Posted by brucepick

This version of the template drawing shows two ways to meet the template without going past the rear bumper.

Aero wizards - what do you see as the plusses/minuses of these two?

Here are my poorly educated guesses:
Blue line is intended to be horizontal. Would allow a fairly clean view out the rear window. I think I prefer this version but consider the purple line one also.

Purple line would add downforce?? And I believe generating that downforce creates its own additional amount of drag? Or are the added downforce and drag very small due to relatively low pressure in the area just above the trunk lid?

I'm not an aero-wizard like aerohead, but I think this is good. I would also employ the blue-line solution. Along California98Civic's comment, here is an example for a hatchback that is similar to an XR4TI spoiler, albiet not a biplane (cosworth?!?!?!?) :



The blue-line solution could be a custom spoiler or maybe an adaption of an existing adjustable spoiler.

Here's another example that fits the streamline template (but extends beyond the bumper) :



CarloSW2

[California98Civic]

Originally Posted by cfg83

I'm not an aero-wizard like aerohead, but I think this is good. I would also employ the blue-line solution. Along California98Civic's comment, here is an example for a hatchback that is similar to an XR4TI spoiler, albiet not a biplane (cosworth?!?!?!?) :



The blue-line solution could be a custom spoiler or maybe an adaption of an existing adjustable spoiler.

Here's another example that fits the streamline template (but extends beyond the bumper) :

These are cool, and I begin to see Brucepic's point about the blue line better. Hmmm...

[Pendragon]

With the plate/spoiler angled up to intersect "the template" with a shorter overhang of the rear of the car as shown in the diagram, I have to wonder just how much additional drag will be created.

Out of curiosity, try moving "the template" forward. I think you will find the result interesting. I think, but it is just my opinion that "the template" misplaces the rear because of the choice of both the airfoil and the position of the leading edge, but that is just my opinion.

If you take a look at the current Subaru WRX five door which has an aero add-on at the top of the rear window and compare it with the four door notch back I think you will notice that there is a trend toward establishing a sharper separation point which basically creates a Kamm back on two box designs. The wing on the notchback is probably more of a styling feature to look like the actual rally cars than something which actually does something beneficial at normal highway speeds, but who knows. It is all a matter of drag and few of us have the capability to quantify it on different model designs.

I have seen ads for add-on affairs for SUVs which have a narrow slot to allow air to get to the underside of the wing at the top of the vehicle which were supposed to help keep the rear window clear and improve the air flow in general. As with all such things, a fair degree of skepticism is probably in order.

Chevy has been running ads for its Equinox, which uses a similar approach, claiming that it has the same Cd as a Corvette ZO6. (The truth is that they are both about 0.34-0.36 which isn't all that wonderful, but sounds nice and besides it is an improvement for a SUV.) One or the other of their ads even talks about the Kamm back aerodynamic treatment. Once again, it is as much marketing as anything else, but there is some truth to it.

The Subaru with the trunk mounted wing does illustrate how you could mount some end plates on the trunk lid and position things pretty much however you wanted. The ones on the Bonneville cars are more in line with what I would think to be low drag designs though.

Cheers

[California98Civic]

Originally Posted by Pendragon

With the plate/spoiler angled up to intersect "the template" with a shorter overhang of the rear of the car as shown in the diagram, I have to wonder just how much additional drag will be created

Well, Pendragon, wonder no more, because I just tested it. Drag and/or downforce added by the angle might just be what caused a decline in MPG. Across my 18 mile "course" at 16.5 throttle steady:

A (no mod): 51.7
B (mod): 50.4
A (no mod): 52.0

This was so dramatic a difference I could see it just a few miles after starting out. Slower speeds were another obvious indicator. Clearly, this design does not work, possibly for the reasons you suggest, though there are others here who might know more and might comment. But I will say that at 60mph it was difficult to keep the leading edge of the assembly taped flat onto the trunk lid surface. Even though it was just 2-3" behind the base of the rear window, it would separate and flap in the wind like a parachute. I taped it down hard with duct tape and started again. I should have tuft tested first. The air does not move the way I thought.

Next, I want to try a less aggressive approach, combining ideas you (Pendragon) and Brucepic and probably others have already suggested. That tiny "kamm" type add-on at the top of the window (for cleaner separation) and a small, flat spoiler that does not extend beyond the bumper and that is flat or slightly angled downward.

EDIT: Added previous long flat design photo, the over lay of both angles, and flat spoiler test results on the long flat spoiler, for reference: http://ecomodder.com/forum/showthrea...tml#post241134

[COcyclist]

Originally Posted by California98Civic

Next, I want to try a less aggressive approach, combining ideas you (Pendragon) and Brucepic and probably others have already suggested. That tiny "kamm" type add-on at the top of the window (for cleaner separation) and a small, flat spoiler that does not extend beyond the bumper and that is flat or slightly angled downward.

Thanks for testing these ideas. I am considering something similar to this. Are you thinking that the small, flat spoiler would be like the Merkur spoiler or the "blue line" posted above, so it meets the template?

[California98Civic]

Originally Posted by COcyclist

Thanks for testing these ideas. I am considering something similar to this. Are you thinking that the small, flat spoiler would be like the Merkur spoiler or the "blue line" posted above, so it meets the template?

My thought is to abandon the template and do something modest so as not to disturb the flow gods too much. Something like the attached photo of a Mercedes I saw a few weeks ago, though my trunk lid spoiler would be a little longer and flatter. All I want is cleaner separation. If I can get that, I think I am done.